The calculations are pretty simple. Look at how much your electricity provider is charging per kilowatt hour. Depending on the time of day and season that cost can vary a bit. Lets say your provider is charging 15 cents per kilowatt hour and your Chevy Spark EV or any other EV is getting 4.2 miles per kilowatt. 0.15 cents divided by 4.2 miles is 0.0357 cents per mile. Round that up and you get $0.04 per mile in electricity cost if you charge from home. Now lets say your gasoline car gets 35 miles per gallon on average. Average price in the United States today for gasoline is $3.54 per gallon. Take $3.54 and divide that by 35 miles and you get $0.10 per mile.
So an EV charged at home is less than half the cost per mile than a gasoline car to run per mile. Now 15 cents per kilowatt hour is pretty expensive and if you charge between midnight and 8 AM is usually is more like 10 cents per kilowatt. Many electricity providers will give you an additional penny off per kilowatt if you install a 220 or 240 EVSE charger in your garage from a list of EVSE chargers that has a feature to let them turn off the charger in the event they need to lessen the load on the grid. So under the right circumstances an EV that gets 4.2 miles per kilowatt can be as cheap as $0.02 per mile in electricity costs. I find that $0.04 is a good rule of thumb.
Now depending on how and where you drive it is not uncommon to see your Spark EV get 4.6 to 5.3 miles per kilowatt. It will be even cheaper per mile. The average SUV only gets 26.4 mpg so the cost per mile goes up significantly.